Duncan is a town in Greenlee County, Arizona, United States. As of the 2010 census, the population of the town was 696. In 2018 the estimated population was 789.

Duncan is in the Gila River valley, 4 miles (6 km) west of the Arizona-New Mexico border. The town limits are on both sides of the Gila, but the primary portion of the town and the entire downtown area lie on the south side of the river. Duncan was founded in the mid 19th century, and the land was added to the United States as a part of the Mexican Cession. The town of Duncan has been destroyed twice by flood and once by fire.

The town and area are primarily populated by ranchers and miners (especially from the Freeport-McMoran copper mines in southeastern Arizona and southwestern New Mexico). Surrounding smaller towns such as Franklin and York in Arizona and Virden, New Mexico, use Duncan public works and public schools. The Duncan area along the Gila River is renowned for Native American artifacts such as arrow heads, pottery, burial sites, cave paintings and other remnants of the Anasazi and other pre-historic cultures, as well as artifacts from garrison camps of the expedition of Francisco Vázquez de Coronado.

Duncan High School (mascot: Wildkats; school colors: red, grey, and white) competes in many sports, but is renowned for their football team.

U.S. Supreme Court Justice Sandra Day O’Connor was born in El Paso, Texas, but grew up near Duncan on the Lazy B ranch, which straddles the border between Arizona and New Mexico. The Day family ran the ranch for many years until selling it; it continues to be run as a ranch. O’Connor later wrote a book titled Lazy B: Growing up on a Cattle Ranch in the American Southwest about her childhood experiences on the ranch with her brother H. Alan Day.